1. Field of the Invention
The present invention generally relates to the offshore production of oil and gas. More particularly, it concerns dry-tree, vertical risers supported by semisubmersible vessels.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
A semi-submersible is floating unit with its deck(s) supported by columns to enable the unit to become almost transparent for waves and provide favorable motion behavior. The unit stays on location using dynamic positioning and/or is moored by means of catenary mooring lines terminating in piles or anchors in the seafloor. A DeepDraftSemi® platform is a semi-submersible unit fitted with oil and gas production facilities in ultra deep water conditions. The unit is designed to optimize vessel motions to accommodate steel catenary risers (SCRs)—steel pipes hung in a catenary configuration from a floating vessel in deep water to transmit flow to or from the sea floor.
The “christmas tree” (or “tree”) is an assembly of valves at the top of the tubing of a completed well that are used to control the flow of oil and/or gas and to enable certain manipulations. If the christmas tree is at the level of the seabed, the well is described as “subsea completed” or “wet tree.” If the tree is on the deck of a platform, the well is described as “surface completed” or “dry tree.”
A dry tree semi (DTS) is a floating facility carrying surface-completed wells, i.e. the christmas trees are located above the surface of the sea, on the semi-submersible, as opposed to the seabed.
The rigid pipes (tubing, casing, etc.) that link the trees to the wells require high tension to avoid buckling. The DTS is therefore under constant tension to compensate for the heave motion of the vessel.
Generally, a DTS also carries basic drilling equipment to allow down-hole intervention on a tender assist mode. It may also feature full drilling capability.
A well bay is an area of an offshore platform where the christmas trees and wellheads are located. It normally consists of two levels, a lower level where the wellheads are accessed and an upper level where the trees are accessed often along with the various well-control panels, which typically have pressure gauges and controls for the hydraulically actuated valves, including downhole safety valve and annular safety valve. On a platform with a drilling package, the well bay will be located directly below it to facilitate access for drilling and well interventions.
Spar type platforms have incorporated a conductor and a keel joint centralizer when using air cans for riser tensioning. These conductors are large and part of the air can assembly. Installation or removal requires a heavy-lift vessel for handling. These systems generally have steel-on-steel contact for the keel guide, and therefore impart large axial tension variations to the risers. Alternatively, hydro-pneumatic tensioners have been used to tension the risers. Each known example of these systems has had four cylinders per riser.
Tension configuration (hanging cylinders) have been used on six-cylinder configurations on certain tension leg platforms and on deepwater drilling vessels using the N-Line™ direct acting riser tensioning system (National Oilwell Varco, Houston, Tex. 77036).
U.S. Pat. No. 6,648,074 to Finn et al. describes a gimbaled table riser support system for a spar type floating platform having risers passing vertically through the center well of a spar hull. The gimbaled table is supported above the top of the spar hull. The table is supported by a plurality of non-linear springs attached to the top of the spar hull. The non-linear springs compliantly constrain the table rotationally so that the table is allowed a limited degree of rotational movement with respect to the spar hull in response to wind- and current-induced environmental loads. Larger capacity non-linear springs are located near the center of the table for supporting the majority of the riser tension, and smaller capacity non-linear springs are located near the perimeter of the table for controlling the rotational stiffness of the table. The riser support table comprises a grid of interconnected beams having openings through which the risers pass. The non-linear springs may take the form of elastomeric load pads or hydraulic cylinders. The upper ends of the risers are supported from the table by riser tensioning hydraulic cylinders that may be individually actuated to adjust the tension in and length of the risers. Elastomeric flex units or ball-in-socket devices are disposed between the riser tensioning hydraulic cylinders and the table to permit rotational movement between the each riser and the table.
U.S. Pat. No. 7,013,824 to Otten et al. discloses a riser centralizer for transferring lateral loads from the riser to a platform hull which includes a keel centralizer mounted on a keel joint. The keel centralizer is received within a keel guide sleeve secured in a support mounted at the lower end of the platform hull. The keel centralizer includes a nonmetallic composite bearing ring having a radiused peripheral profile for minimizing contact stresses between the keel centralizer and the keel guide sleeve in extremes of riser and platform motion. The internal surface of the keel guide sleeve is clad with a corrosion resistant alloy and coated with a wear resistant ceramic rich coating.
U.S. Pat. No. 7,632,044 to Pallini et al. describes a ram style tensioner with a fixed conductor and a floating frame. The riser tensioner for an offshore floating platform has a frame mounted to the upper portion of the riser. Pistons and cylinders are spaced circumferentially around the riser and connected between the frame and the floating platform. A tubular guide member is mounted to the floating platform for movement in unison in response to waves and currents. The riser extends through the guide member. A guide roller support is mounted to and extends downward from the frame around the guide member. A set of guide rollers is mounted to the guide roller support in rolling engagement with the guide member as the guide member moves in unison with the platform.
U.S. Pat. No. 8,123,438 to Pallini et al. describes a ram style tensioner that includes a frame configured to be fixedly attached to the riser; plural cylinder assemblies spaced around the riser, each cylinder assembly having a cylinder and a piston configured to slidably move inside the cylinder, the piston being configured to connect to the frame; a guide roller support stationarily mounted to and extending from the frame; at least one bearing fixedly attached to the guide roller support; and a guide member configured to be in rolling engagement with the at least one bearing as the cylinder moves relative to the frame.
U.S. Pat. No. 7,588,393 to Shivers et al. describes a method for supporting top-tensioned drilling and production risers on a floating vessel using a tensioner assembly above the waterline of the vessel. The method may include attaching at least one hydraulic cylinder on a first end to a first position on a floating vessel and on a second end to a tension frame below the first position. The next step of the method may be forming a fluid connection between the at least one hydraulic cylinder and at least one primary accumulator.
U.S. Pat. No. 7,886,828 to Shivers et al. describes a floating vessel for supporting top tensioned drilling and production risers having a hull and an operation deck disposed on top of the hull. The tensioner assembly moveably carries a conductor that communicates from a wellhead to a piece of well access equipment. The well access equipment is connected to the floating vessel. The tensioner assembly is supported by the floating vessel.
For a Dry Tree Semi (DTS) platform, a tensioning system is needed that can provide large strokes (on the order of 30 to 45 feet) and also provide sufficient support and alignment to the risers. Connecting jumpers of production riser christmas trees and drilling riser blowout preventers (BOP's) must be free to move as required by the platform motions without impacting deck or tensioning system components while preventing riser clashing. In addition, the semi-submersible configuration lends itself to a two-main-deck configuration and, due to the tensioner stroke required and the need for access to the christmas trees, tension joints, and BOP's, the tensioning system preferably has a ram or push-up type configuration. By using a push-up tensioner, the tensioner cylinder barrel may be located lower on the deck and enable access to critical areas of the system such as the tension ring and surface trees. In addition, the push-up type arrangement allows for a more compact well bay.
However, a ram type or push-up configuration is susceptible to buckling failure and high lateral loads. What is needed is a method that provides stability to the riser and tensioner while not adversely affecting the low tensioner spring rate that may be required by the DTS design parameters. A keel guide system for the riser is needed to react lateral riser loads directly to the hull structure rather than supporting high riser lateral loads at the tensioner and deck interface. Reacting riser lateral loads at the pontoon level of a semi-submersible may also improve the overall stability of the platform.